To better understand the antigenic nature and the immune response to P. carinii, we have undertaken to characterize the major surface antigen of both rat and human P. carinii. It is necessary to use P. carinii from both sources because antigenically they are different and specifically the major surface antigen in rat and human are clearly different. We have previously purified the major surface glycoprotein (MSG) of both rat and human pneumocystis using HPLC. Subsequently, we identified a number of clones from a cDNA library of P. carinii that contain genes encoding for the MSG. These clones are clearly related but not identical, demonstrating that multiple genes encode the MSG. By pulse-field gel electrophoresis, these clones hybridize to all chromosomes. Predicted protein is high in cysteine and virtually all cysteine residues are conserved in all the clones. Over the past year we have continued studies to characterize the genetic organization and regulation of this family of genes. We have found that these genes are organized as tandem repeats, and that a single or limited number of sites regulate expression of the genes. Our collaborators at UCSF have demonstrated that a single upstream conserved sequence encoding a leader peptide is potentially expressed with each variable downstream MSG sequence. Over the past year we have cloned a number of human P. carinii MSG genes and are currently attempting to produce recombinant protein clone and express some of these genes to allow studies of immune responses to P. carinii in humans. The goal of this study is to better understand the pathogenesis of P. carinii pneumonia with the hope that we can use this information to control or prevent this disease.